Long lasting, high power density power sources are important to enable emerging technologies such as wireless sensor networks, robotic platforms, and electronic devices for consumer, military, medical, aerospace and other applications. To meet the energy demands for these applications, devices that scavenge power from the environment (e.g., solar, thermal, vibrations) are of great practical interest. Various energy harvesting and scavenging methods exist for capturing and storing energy from normally occurring environmental sources, such as thermal, solar, or vibrational. For applications on moving platforms, vibrational energy harvesters are advantageous since solar or thermal energy may not be available under all operating conditions.
Current research has focused on a variety of vibrational energy harvesting devices. For example, micromachining and micro-electro-mechanical system (MEMS) technologies have been used to produce sub-millimeter microchip-sized devices, but the power output from these miniaturized devices has been very low (often nW-μW level), which appears to be too small to power many practical devices. The paper entitled “Performance limits of the three MEMS inertial energy generator transduction types,” by P. D. Mitcheson, et al. (J. Micromech. Microeng., vol. 17, S211-S216, 2007) shows that the power density scales unfavorably with length scale.
FIG. 1A shows an example of a vibrational energy harvester by PMG Perpetuum. As shown in FIG. 1A, the vibrational energy harvester is almost the size of an apple, thus the low power density provided by vibrational energy harvesters creates a trade-off between size and power. Specifically, to achieve higher power, the size of the harvester needs to be larger. FIG. 1B shows a schematic of an inertial vibrational energy harvester, which can be piezoelectric, capacitive, or magnetic.
As noted by S. P. Beeby et al. in “Energy harvesting vibration sources for microsystems applications,” (J. Measurement Science and Technology, vol. 17, pp. R175-R195, 2006) and D. P. Arnold in “Review of microscale magnetic power generation,” (IEEE Trans. Magn., vol. 43, no. 11, pp. 3940-3951, 2007), devices that use electromagnetic transduction schemes have generally shown higher power densities (up to 2 mW/cm3) when compared to electrostatic and piezoelectric approaches.
Most of the current systems are fairly high-Q resonant mass-spring-damper style devices designed for maximum performance at only one narrowly defined frequency. In fact, most resonant devices can operate at only one single frequency, but many naturally occurring vibrations have broadband frequency content. This narrowband frequency response is especially problematic for micromachined devices, which typically possess resonant frequencies in excess of 1 kHz, well above the frequency range of mechanically- or human-induced vibrations (1-500 Hz).
Additionally, most current systems only respond to one axial direction of motion. For many naturally occurring vibrations, e.g. human motion, vehicle motion, energy harvesters are desired that can capture the complex six-degree-of-freedom linear and rotational motions.
Moreover, prototypes have successfully demonstrated electrical power extraction, but conversion and regulation of the extracted electrical power to the appropriate voltage/current levels for compatibility with electronic devices continues to present engineering challenges. Conventional power electronic circuit approaches currently do not appear to function efficiently at the low voltages and currents supplied by a typical vibrational energy harvester.